Recently, there has been a trend toward downsizing and high power in resin parts for electrical or electronic use for automobiles and the like. With this trend, the heat dissipation properties of such resin parts have been important factors. For this reason, there is a demand for resin parts whose thermal conductive properties are greatly improved with electrical insulating properties thereof being maintained. In addition, as the thicknesses of such resin parts are reduced for the downsizing and the heat dissipation improvement of the resin parts, resin parts having a good balance of high levels of strength, rigidity, and impact resistance have been demanded.
Conventionally known electrically insulating resin materials having high thermal conductive properties include those obtained by adding to a resin a large amount of an electrically insulating filler having high thermal conductive properties, such as alumina or boron nitride. However, the addition of an electrically insulating filler having high thermal conductive properties in a large amount causes a problem of deterioration in strength and impact resistance and a problem of deterioration in molding processability. Moreover, since the fillers having high thermal conductive properties are expensive, there is room for improvement in production costs.
Japanese Unexamined Patent Application Publication No. 2002-188007 (PTL 1), Japanese Unexamined Patent Application Publication No. 2003-40619 (PTL 2), and International Application Japanese-Phase Publication No. 2007-517968 (PTL 3) disclose resin composite materials and resin compositions each comprising a resin and calcium fluoride, and also disclose that these resin composite materials and resin compositions are excellent in thermal conductive properties and moldability. However, these resin composite materials and resin compositions do not have a good balance of high levels of thermal conductive properties, strength, rigidity, and impact resistance, and are insufficient as materials for resin parts for electrical or electronic use for automobiles and the like.
Meanwhile, since carbon-based nanofillers represented by carbon nanotubes are excellent in thermal conductive properties, addition of carbon-based nanofillers to resins have been examined to impart thermal conductive properties to the resins or to improve thermal conductive properties of the resins. The addition of such a carbon-based nanofiller tends to improve the thermal conductive properties. However, the electrical conductive properties are increased, because an electrical conductive path is formed due to contact of pieces of the carbon-based nanofiller in the resin. Hence, it is difficult to obtain a resin composition having high levels of thermal conductive properties and electrical insulating properties simultaneously.
In this respect, the present inventors have proposed a resin composition comprising a carbon-based nanofiller, a modified polyolefin-based polymer, and two or more resins as a resin composition having high levels of thermal conductive properties and electrical insulating properties simultaneously (Japanese Unexamined Patent Application Publication No. 2010-100837 (PTL 4)). In the case of the resin composition, the addition of the modified polyolefin-based polymer makes it possible to localize the carbon-based nanofiller in one of the two or more resins, the one resin forming a dispersed phase. This enables improvement in thermal conductive properties and electrical insulating properties to high levels in a balanced manner. The PTL 4 also discloses that the thermal conductive properties are further improved by using, in combination, a filler having high thermal conductive properties such as alumina and boron nitride.
However, resin parts for electrical or electronic use for automobiles and the like have been required to have higher electrical insulating properties, and also to have high levels of strength, rigidity, and impact resistance simultaneously.